In a process of forming an image display such as a liquid crystal display, various optical films for forming the display, such as polarizing plates and retardation plates are attached to an object such as a liquid crystal cell with a pressure-sensitive adhesive layer interposed therebetween. The pressure-sensitive adhesive layer is often previously provided on one side of the optical film, because it is advantageous in that the optical film can be instantly fixed onto a display panel such as a liquid crystal cell and that no drying process is necessary to fix the optical film.
Essential properties required of the pressure-sensitive adhesive include the stably adhesive characteristic for the optical films attached to a liquid crystal cell or the like during the life-span of an image display. On the other hand, even when misalignment occurs or foreign matter is caught on the attaching surface in the process of attaching an optical film (the adherend) to a liquid crystal cell or the like, the optical film should be reworkable so that it can be peeled off from the liquid crystal cell and attached again. In view of environment countermeasure such as recycling or disposal, the optical film should be easily peeled off from the liquid crystal cell or the like at the end of the life of the image display.
A glass substrate designed specifically for use in a liquid crystal cell has a thickness of about 0.7 mm. Since the area of liquid crystal displays themselves has been steadily increasing in the field of televisions and monitors, breakage such as breakage of the glass substrate during the peeling off of the optical film can more frequently occur. In addition, a glass substrate for use in a mobile liquid crystal display is reduced in thickness by etching in terms of reducing the weight and the thickness. Such a glass substrate is thinner than 0.4 mm, which makes difficult the peeling off of optical films.
There is a trade-off between the adhesive characteristic and the easy peelability. If one of these properties is satisfied, the other will be degraded. If the pressure-sensitive adhesive layer sufficiently offers the adhesive characteristic, and, for example, if an increased adhesive strength is provided during reworking or particularly after adhesion for a long time, the process of peeling off an optical film from a display panel may result in breakage of the display panel or the like. On the other hand, if the adhesive strength of the pressure-sensitive adhesive layer is reduced so that the peelability can be high during reworking or at the end of the life of the image display, a problem with adhesion reliance may occur, such as peeling of the optical film during the operation of the image display.
Particularly when optical films such as polarizing plates (polarizers and transparent protective films), optical compensation films and brightness enhancement films (reflective polarizer) are used with a pressure-sensitive adhesive or an adhesive as needed to form a laminated structure in an image display, the total thickness of the optical films may reach about ⅕ to about ½ of the thickness of the image display. Also in view of recycling of display panel materials, therefore, peeling off of optical films from a display panel composed mainly of glass has been desired.
When the image display is used in a normal environment, such optical films serve to impart toughness so that the display panel produced with glass can be prevented from breaking. Therefore, conventional pressure-sensitive adhesives have been designed based on adhesion reliance and not designed from the viewpoint of recycling, although it is very important to peel off optical films from the display panel in the process of recycling the image display. Particularly after the display panel is used for a long time, the optical films are attached to the display panel by a strong force based on the adhesive strength of the pressure-sensitive adhesive, and in general cases, the adhesive strength increases with time. Therefore, when the image display is recycled, the glass part of the display panel frequently breaks in the process of peeling off the optical films from the display panel, which is not preferred for recycling and poses a risk of injury by glass fragments.
There is a method of recycling an image display, which includes peeling off an optical film using a solvent capable of dissolving a pressure-sensitive adhesive. In general, however, solvent resistance is imparted to the optical film for use, and therefore, in the recycling method with a solvent, it is difficult to allow the solvent to sufficiently reach and dissolve the pressure-sensitive adhesive layer covered with the optical film. Thus, the recycling method with a solvent needs a large amount of solvent and a long time. In the recycling method with a solvent, it is also difficult to successfully discard the solvent in which the pressure-sensitive adhesive is dissolved, which may cause significant problems with efficiency and environmental issues.
In order to achieve both the adhesive characteristic and the easy peelability for reworking, it is proposed to add a block polyisocyanate compound to a pressure-sensitive adhesive (Patent Document 1). Patent Document 1 discloses that the block polyisocyanate compound causes crosslinking at a temperature of 120° C. or more so that the adhesive strength can be reduced to such a level that peeling off can be easily performed. However, it is also expected that if the block polyisocyanate compound is heated at a relatively low temperature of 80° C. under actual operation conditions, a certain degree of dissociation may be caused by crosslinking, so that the adhesive strength of the pressure-sensitive adhesive layer may be reduced and separation or peeling of the optical film may occur before the end of the life of the image display. There is also concern that the decomposition of the block polyisocyanate compound may generate poisonous gas such as phenol or amine to affect the environment.
It is also proposed to add a solid foaming agent or a microcapsule foaming agent to a pressure-sensitive adhesive (Patent Document 2). Patent Document 2 discloses that gas is generated from the foaming agent by heating to reduce the contact area with the adherend, so that the adhesive strength is reduced. Concerning the solid foaming agent, however, ammonium carbonate disclosed as an example is decomposed at a low temperature (58° C.) and therefore insufficiently stable over time. It is also considered that since the solid foaming agent is gradually decomposed over a long time, gas cannot sufficiently be generated even after the end of the life of the image display, so that the adhesive strength cannot be reduced. The ammonium carbonate also has the problem of the generation of carbon dioxide, which puts stress on environment, and toxic ammonia. The microcapsule foaming agent is also not preferred, because the capsule size cannot be reduced, so that light scattering may occur to impart a haze to the pressure-sensitive adhesive itself. It is also considered that if the capsule does not have uniform strength, foaming may occur at a temperature lower than the original foaming temperature.
Besides the above, it is also proposed that particles with an average particle size of 0.5 to 15 μm should be dispersed in a pressure-sensitive adhesive (Patent Document 3). However, the function of the particles dispersed in the pressure-sensitive adhesive disclosed in Patent Document 3 is only to scatter the light passing through the pressure-sensitive adhesive layer. Therefore, the adhesive strength of the pressure-sensitive adhesive cannot be controlled using such dispersed particles.